Oxygen doping method to gallium nitride single crystal substrate and oxygen-doped N-type gallium nitride freestanding single crystal substrate

ABSTRACT

Oxygen can be doped into a gallium nitride crystal by preparing a non-C-plane gallium nitride seed crystal, supplying material gases including gallium, nitrogen and oxygen to the non-C-plane gallium nitride seed crystal, growing a non-C-plane gallium nitride crystal on the non-C-plane gallium nitride seed crystal and allowing oxygen to infiltrating via a non-C-plane surface to the growing gallium nitride crystal.  
     Otherwise, oxygen can be doped into a gallium nitride crystal by preparing a C-plane gallium nitride seed crystal or a three-rotationally symmetric plane foreign material seed crystal, supplying material gases including gallium, nitrogen and oxygen to the C-plane gallium nitride seed crystal or the three-rotationally symmetric foreign seed crystal, growing a faceted C-plane gallium nitride crystal having facets of non-C-planes on the seed crystal, maintaining the facets on the C-plane gallium nitride crystal and allowing oxygen to infiltrating via the non-C-plane facets to the gallium nitride crystal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an oxygen doping method into a galliumnitride crystal and an oxygen-doped n-type gallium nitride singlecrystal substrate for producing light emitting diodes (LEDs), laserdiodes (LDs) or other electronic devices of groups 3 and 5 nitridesemiconductors. Nitride semiconductors means GaN, InGaN, InAlGaN and soon which are grown as thin films on a sapphire substrate. An activationlayer is a GaInN layer. Other parts are mainly GaN layers. Thus, thelight emitting diodes based upon the nitride semiconductors arerepresented as GaN-LEDs or InGaN-LEDs which mean the same LEDs.

[0003] This application claims the priority of Japanese PatentApplication No. 2001-113872 filed on Apr. 12, 2001 which is incorporatedherein by reference.

[0004] 2. Description of Related Art

[0005] Light emitting devices making use of nitride semiconductors havebeen put on the market as blue-light LEDs. At present, all of theavailable nitride light emitting devices are made upon sapphiresubstrates. An epitaxial wafer is obtained by growing a GaN film, aGaInN film and so forth upon a C-plane single crystal sapphire substrateheteroepitaxially. A unique n-dopant for GaN, AlInGaN, or InGaN thinfilms is silicon (Si). Silicon acts as an n-impurity in GaN by replacinga gallium site. A series of wafer processes produces GaInN-LEDs on theon-sapphire epitaxial wafer. A lattice constant of sapphire (α-Al₂O₃) isdifferent from that of gallium nitride. Despite the large latticemisfit, a sapphire substrate allows gallium nitride to growheteroepitaxially on it. The on-sapphire GaN includes great manydislocations. In spite of the many dislocations, the GaN films onsapphire are stable and endurable.

[0006] Sapphire is a crystal of a trigonal symmetry group. C-plane ofsapphire has quasi-three fold rotation symmetry. Gallium nitride belongsto hexagonal symmetry. C-plane of gallium nitride has perfect three-foldrotation symmetry. Since the symmetry groups are different for GaN andsapphire, any other planes than C-plane of sapphire cannot grow a GaNcrystal. Thus, the GaInN-LEDs in use include sets of c-axis grown InGaN,InGaAlN or GaN thin films grown on C-planes of sapphire substrates.

[0007] All the GaN or GaInN thin films heteroepitaxially grown on thesapphire substrates are C-plane growing crystals. Sapphire substratescannot make non-C-plane growing GaN crystals at all. Since sapphire hasbeen a unique seed crystal for growing GaN until recently, it has beenimpossible to make a non-C-plane GaN crystal. At present, all theGaInN-LEDs and GaInN-LDs on the market consist of a pile of C-planegrown GaN, InGaN or AlInGaN thin films grown on C-plane sapphiresubstrates.

[0008] Large lattice misfit between sapphire and gallium nitride inducesplenty of dislocations in a gallium nitride crystal grown on a sapphiresubstrate. Gallium nitride has rigidity as high as ceramics. Therigidity maintains the framework of crystals for a long time. Inherentdislocations in GaN crystals of LEDs do not enlarge by current injectionunlike GaAs crystals. Since the dislocations do not increase, the GaNcrystals on sapphire do not degrade. In spite of the great manydislocations, GaN-LEDs enjoy a long life time, high reliability and goodreputation.

[0009] Sapphire substrates, however, have some drawbacks. Sapphire is avery rigid, hard crystal. Sapphire lacks cleavage. Sapphire is aninsulator. Rigidity, non-cleavage and insulation are weak points ofsapphire. When a plenty of device units have been fabricated upon asapphire substrate wafer by wafer processes, the device-carryingsapphire wafer cannot be divided by natural cleavage like siliconwafers. The sapphire wafer should be mechanically cut and divided intoindividual device chips. The mechanical dicing step raises the cost.

[0010] The non-cleavage is not a serious obstacle for making LEDs (lightemitting diodes) on sapphire substrates, since an LED has no resonatormirror. In the case of producing LDs (laser diodes) on sapphiresubstrates, the non-cleavage is a fatal drawback. A laser diode (LD)requires two mirrors at both ends of an active (stripe) layer as aresonator for amplifying light by injected current. It is convenient toform resonator mirrors by natural cleavage in a laser diode, becausenatural cleaved planes are endowed with flatness and smoothness.On-sapphire LDs prohibit natural cleavage from making resonator mirrors.Flat, smooth mirrors should be made on both ends of the laser chips by avapor phase etching method, e.g., RIE (reactive ion etching), which is adifficult operation. Mirror-polishing should be done chip by chip afterthe wafer process has finished. Mirror-finishing of the resonators bythe RIE is a main reason raising the cost of manufacturing theon-sapphire GaInN-LDs.

[0011] Another drawback results from the fact that sapphire is aninsulator. Insulation prevents on-sapphire LEDs and LDs from having ann-electrode on the bottom. Sapphire forces LEDs and LDs to have extran-type layers upon an insulating substrate but below an active layer.The n-electrode is formed by partially etching away a p-layer and theactive layer, revealing the extra n-layer and depositing an n-electrodealloy on the n-layer. Both a p-electrode and the n-electrode are formedon the top surface of the LED or LD. Since electric current flows in thehorizontal direction, the n-layer should have a sufficient thickness. Ittakes much time to eliminate a part of the p-layer and form an ohmicn-electrode on the revealed n-layer. An increase of the steps and timeenhances the cost of the on-sapphire LEDs. Both the n-electrode and thep-electrode occupy a wide area on the top of the LED, which raises anecessary area of the LED. On-sapphire GaInN-LEDs which are prevailingcannot conquer the above drawbacks yet.

[0012] A gallium nitride (GaN) single crystal substrate would be anideal substrate which has a probability of solving the drawbacks. Sincethin films of GaN or GaInN are epitaxially deposited upon a substratefor producing blue light LEDs and LDs, a GaN bulk single crystal wouldeliminate the problem of lattice misfitting between the deposited filmsand the substrate. If an n-type bulk single crystal GaN substrate can beproduced, an n-electrode can be formed on the bottom of the n-type GaNsubstrate. An allocation of a p-electrode at the top and an n-electrodeat the bottom facilitates to produce LEDs, to bond the LEDs on packages,and to wirebond the LEDs to wiring patterns on the packages. The bottomn-electrode enables an LED to reduce the chip size.

[0013] Another advantage results from cleavability of a bulk GaN singlecrystal. A device-produced GaN wafer can be divided into stripe arraysof individual device (LED or LD) chips by natural cleavage. However,cleavage planes (1-100), (01-10) and (-1010) are parallel to three sidesof an equilateral triangle defined upon a C-plane (0001) of GaN. The GaNcrystal has not a square set of cleavage planes but a triangle set ofcleavage planes. Square device (LED or LD) chips are produced by cuttinga device-carrying GaN wafer partially by natural cleavage and partiallyby mechanical dicing.

[0014] Furthermore, an LD (laser diode) chip can produce resonatormirrors by natural cleavage. Replacement of the RIE by the naturalcleavage reduces the cost of making GaInN-type blue light laser diodes(LDs).

[0015] However, there is no mineral containing gallium nitride singlecrystals. No attempt of making a wide, bulk GaN single crystal substrateartificially has succeeded until recently. Since a GaN single crystalsubstrate was inaccessible, it was not possible to make GaInN type LEDsor LDs on a single crystal GaN substrate until recently.

[0016] Recently, vapor epitaxial methods which can grow a GaN singlecrystal on a foreign material substrate have been proposed and improved.The methods are described as follows.

[0017] (1) Metallorganic Chemical Vapor Deposition Method (MOCVD)

[0018] The most prevailing method for making GaN crystals is aMetallorganic Chemical Vapor Deposition Method (MOCVD). The MOCVDproduces a GaN crystal by placing a sapphire substrate in a cold-wallfurnace, heating the sapphire substrate, supplying a material gasincluding TMG (trimethylgallium) and ammonia (NH₃) to the sapphiresubstrate, and synthesizing gallium nitride (GaN) from the material gason the substrate. Although a great amount of the material gas is inhaledinto the furnace, only a part of the material gas reacts with each otherfor making gallium nitride molecules. Other part of the material gas isdissipated in vain. The MOCVD is suffering from low yield and lowgrowing speed. The MOCVD is favorable for making thin GaN films but isunsuitable for producing a thick GaN crystal layer due to the materialdissipation. Another drawback is possibility of carbon contamination dueto carbon included in the metallorganic gases.

[0019] (2) Metallorganic Chloride Method (MOC)

[0020] An MOC method produces a GaN crystal by placing a sapphiresubstrate or GaAs substrate in a hot-wall furnace, supplying TMG(trimethylgallium) and HCl (hydrochloric acid) into the furnace,synthesizing GaCl (gallium chloride) above the substrate, supplyingammonia (NH₃) to the heated substrate, inducing a reaction between NH₃and GaCl on the substrate, making gallium nitride molecules on thesubstrate and depositing the gallium nitride molecules on the substrate.Since the MOC method makes once an intermediate compound GaCl, carboncontamination is alleviated in comparison with the MOCVD. However, theMOC is not fully immune from possibility of carbon contamination, sincethe MOC employs trimethylgallium gas.

[0021] (3) Hydride Vapor Phase Epitaxy Method (HVPE)

[0022] Unlike the MOCVD or the MOC, an HVPE employs metal Ga monoelementas a gallium source. FIG. 1 shows a HVPE apparatus having a hot-wallfurnace 1. A heater 2 is upholstered around the furnace 1. Gas inlets 3and 4 are provided at the top of the furnace 1 for introducing two kindsof material gases. The furnace 1 sustains a Ga-boat 5 at an upper space.A Ga-melt 6 is prepared by putting metal Ga into the Ga-boat 5 andheating the Ga-boat 5 by the heater 2. One gas inlet 3 has an open endfacing to the Ga-boat 5 for supplying H₂+HCl gas to the Ga-boat 5. Theother gas inlet 4 has an open end at a middle height of the furnace forintroducing H₂+NH₃ gas.

[0023] A susceptor 7 is supported by a rotation shaft 8 in a lower halfof the furnace 1. The rotation shaft 8 can rotate, lift up or down thesusceptor 7. A GaAs substrate or a GaN substrate 9 is laid upon thesusceptor 7 as a seed. A GaN seed crystal can be prepared by making aGaN crystal on a GaAs substrate, eliminating the GaAs substrate andslicing the grown GaN crystal into wafers. The heater 2 heats thesusceptor 7 and the substrate 9. An intermediate compound galliumchloride GaCl gas is synthesized by blowing the HCl+H₂ gas to theGa-melt 6 in the boat 5. GaCl falls in the furnace near the substrate 9,reacts with ammonia and synthesizes gallium nitride (GaN) on thesubstrate 9. The HVPE uses a non-carbon material (Ga monoelement). TheHVPE is free from possibility of carbon contamination which degradeselectric properties of object crystals.

[0024] (4) Sublimation Method

[0025] Heating alone cannot convert solid GaN into a melt of Ga. Highpressure is required for melting solid GaN besides heating. Difficultyof making a GaN melt prohibits a Czochralski method or a Bridgman methodfrom growing a GaN solid from a GaN melt. Without high pressure, solidGaN sublimes into vapor GaN by heating. A sublimation method makes a GaNsingle crystal on a substrate by inserting a substrate and a GaNpolycrystal source into a reaction tube, heating the GaN polycrystalsource for subliming at a higher temperature, heating the substrate at alower temperature, transporting GaN vapor from the GaN source to thecolder substrate and depositing GaN molecules on the substrate.

[0026] Another improvement (Lateral Overgrowth Method) has been proposedfor making a low-dislocation density GaN film grown on a sapphiresubstrate for making on-sapphire GaInN-LEDs.

[0027] [Epitaxial Lateral Overgrowth Method (ELO)]

[0028] {circle over (1)} Akira Usui, “Thick Layer Growth of GaN byHydride Vapor Phase Epitaxy”, Electronic Information and CommunicationSociety, C-II, vol.J81-C-II, No. 1, pp58-64, (January, 1998), proposed agrowth of GaN by a lateral overgrowth method. The lateral overgrowthmethod produces a low dislocation density GaN crystal by covering asapphire substrate with a mask having dotted or striped windows lying atcorner points of periodically allocated equilateral triangles, supplyingmaterial gas via the mask windows to the sapphire substrate, depositingGaN molecules on the sapphire substrate within the windows, growingfurther GaN films from the windows over on the mask, joining neighboringGaN films in horizontal directions along boundaries between the windowson the mask, turning the growing direction from the horizontaldirections to the vertical direction and maintaining the vertical GaNgrowth on the mask. Dislocations have a tendency of extending along thegrowing direction. Many dislocations accompany GaN growth in any cases.The ELO method forces the dislocations to bend at the meeting boundariesabove the mask from horizontal extensions to an upward extension. Thechange of extension reduces the dislocations in the GaN crystal. Thus,the ELO is effective to reduce dislocation density of a GaN thin filmgrown on a sapphire substrate.

[0029] Inventors of the present invention chose the HVPE method as avery promising candidate among the mentioned vapor phase growth methodsfor growing a thick GaN crystal for a freestanding GaN wafer. Almost allof the preceding trials for growing GaN films have started from sapphiresingle crystals as a substrate. Sapphire has, however, some drawbacks ofnon-existence of cleavage and impossibility of removal. The inventorsabandoned sapphire as a substrate for making a freestanding GaN singlecrystal.

[0030] Instead of sapphire, the inventors of the present invention choseGaAs (gallium arsenide) as a substrate for growing a thick GaN crystalfor making a freestanding GaN single crystal wafer. Although GaAs has acubic symmetry which is different from the hexagonal symmetry of GaN andthe trigonal symmetry of sapphire, a (111) GaAs plane has three-foldrotation symmetry akin to the hexagonal symmetry. The inventors of thepresent invention succeeded in growing a GaN crystal on a (111) GaAssubstrate in the c-direction from materials of metal gallium,hydrogen-diluted hydrochloric acid (HCl) gas and hydrogen-dilutedammonia gas. Fortunately, the GaAs substrate can be eliminated from thegrown GaN crystal by etching or polishing. Possibility of removal is anadvantage of GaAs as a substrate for making a freestanding GaN crystal.

[0031] {circle over (2)} Japanese Patent Application No.10-183446(183446/'98) was filed by the same inventor as the presentinvention. {circle over (2)} produced a GaN single crystal by preparinga GaAs (111) substrate, covering the GaAs substrate with a mask havingwindows, growing a thick GaN layer on the masked GaAs substrate by theHVPE and the ELO method, and eliminating the GaAs substrate by aquaregia. {circle over (2)} obtained a freestanding GaN bulk single crystalwafer having a 20 mm diameter and a 0.07 mm thickness. The GaN crystalwas a C-(0001) plane crystal.

[0032] {circle over (3)} Japanese Patent Application No.10-171276(171276/'98) was filed by the same inventor as the presentinvention. {circle over (3)} also proposed a freestanding GaN bulksingle crystal wafer of a C-plane produced by depositing a thick GaNcrystal upon a (111) GaAs substrate. Distortion of the GaN wafer was aproblem. Distortion is induced on the freestanding GaN wafer bydifferences of thermal expansion between GaAs and GaN. How to reduce thedistortion was another problem for {circle over (3)}. Conduction type ofthe GaN crystal was left untouched.

[0033] {circle over (4)} Kensaku Motoki, Takuji Okahisa, NaokiMatsumoto, Masato Matsushima, Hiroya Kimura, Hitoshi Kasai, KikurouTakemoto, Koji Uematsu, Tetsuya Hirano, Masahiro Nakayama, SeijiNakahata, Masaki Ueno, Daijirou Hara, Yoshinao Kumagai, Akinori Koukituand Hisashi Seki, “Preparation of Large Freestanding GaN Substrates byHydride Vapor Phase Epitaxy Using GaAs as a Starting Substrate”, Jpn. J.Appl. Phys. Vol.40(2001) pp.L140-143, reported a freestanding GaN singlecrystal produced by a lateral overgrowth method upon a GaAs (111)substrate. Grown GaN was a (0001) C-plane crystal having a 500 μmthickness and a 2 inch diameter. The GaN crystal showed n-typeconduction. Dislocation density was 2×10⁵ cm⁻². Carrier density wasn=5×10¹⁸ cm⁻³. Mobility was 170 cm²/Vs. Resistivity was 8.5×10⁻³Ωcm.{circle over (4)} said nothing about n-dopants.

[0034] {circle over (5)} Japanese Patent Application No. 11-144151 wasfiled by the same inventor as the present invention. {circle over (5)}proposed a freestanding n-type GaN single crystal containing oxygen asan n-dopant having nearly 100% of activation rate in GaN. This was thefirst document which asserted that oxygen was a good n-dopant in GaNwith nearly a 100% activation rate. Silicon (Si) has been prevailing asan n-dopant which has been exclusively doped into GaN thin films grownon sapphire substrates in a form of silane gas (SiH₄). But silane gas(SiH₄) is a dangerous gas. Oxygen can be supplied in a safe form ofwater or water vapor to material gases. {circle over (5)} rejectedsilicon but admitted oxygen as an n-dopant in GaN. {circle over (5)}insisted on replacement of silicon by oxygen. {circle over (5)} allegedthat carbon (C) which is an n-impurity and disturbs the action of oxygenshould be excluded from the material gases. {circle over (5)} denied theMOCVD (metallorganic chemical vapor deposition) method which usesmetallorganic gases including plenty of carbon atoms but recommended theHVPE (hydride vapor phase epitaxy) method.

[0035] GaN is a hexagonal symmetry crystal with three-fold rotationalsymmetry. Crystallographical representation of GaN is different fromGaAs(zinc blende type) which belongs to the cubic symmetry group.Crystallographic representation of the hexagonal symmetry group is nowdescribed. There are two representations for hexagonal symmetry. Onemethod uses three parameters. The other method uses four parameters.Here, four parameter representation which requires four axes isdescribed. Three axes are denoted by a-axis, b-axis and d-axis which lieon the same horizontal plane and meet at an origin with each other at120 degrees. Unit lengths a, b and d of the three axes are equal, thatis, a=b=d.

[0036] An extra axis meets with other three axes at 90 degrees. Theextra axis is named c-axis. A set of a-, b-, d-, and c-axes definesplanes and directions in a hexagonal symmetry crystal. The three a-, b-,d-axes are equivalent. But the c-axis is a unique axis. A set of plentyof parallel equivalent planes is imagined. When a first plane crossesa-axis at a point of a/h, b-axis at a point of b/k, d-axis at a point ofd/m and c-axis at a point of c/n, the plane is represented by (hkmn).When the first plane cannot cross a positive part of the axes, the axesshould be extended in a negative direction for crossing with the firstplane. Crystal has a periodic character. Thus, h, k, m and n arepositive or negative integers including zero (0). Number “h” means thenumber of the object planes existing in a unit length “a” of a-axis.Number “k” means the number of the object planes existing in a unitlength “b” of b-axis. The object plane is represented a round bracketedindices (hkmn).

[0037] Three equivalent indices h, k and m for a-, b-, d-axes alwayssatisfy a zero-sum rule of h+k+m=0. The other index n for c-axis is afree parameter. Crystal indices h, k, m and n are substituted in abracket without comma “,”. A negative index should be discriminated froma positive one by upperlining by the crystallograpy. Since an upperlineis forbidden, a negative index is shown by adding a minus sign beforethe integer. There are two index representations. One is an individualrepresentation. The other is a collective representation. Objects of theindex representation are planes and directions. A direction and a planetake the same set hkmn of indices, when the direction is a normal(meeting at 90 degrees) to the plane. But the kinds of brackets aredifferent.

[0038] Round bracketed (hkmn) means an individual representation of aplane. Wavy bracketed {hkmn} means a collective representation of familyplanes. Family planes are defined as a set of planes all of which can beconverted into other member planes by the symmetry operation included inthe crystal symmetry.

[0039] Besides the plane representation, linear directions should bedenoted by a similar manner. Square bracketed [hkmn] means an individualrepresentation of a direction which is vertical to an individual plane(hkmn). Edged bracketed <hkmn> means a collective representation offamily directions. Family directions are defined as a set of directionsall of which can be converted into other member directions by thesymmetry operation of the crystal symmetry. The definitions are shown asfollows. (hkmn) individual, plane. {hkmn} collective, plane. [hkmn]individual, direction. <hkmn> collective, direction.

[0040] In the hexagonal symmetry, C-plane is the most important planerepresented as (0001) which is normal to the horizontal plane includinga-, b- and c-axes. C-plane has three-fold rotational symmetry. All ofthe artificially made GaN crystals have been produced by C-plane growthwhich grows a crystal by maintaining C-plane as a surface. When GaN isheteroepitaxially grown on a foreign material, for example, sapphire(Al₂O₃) or gallium arsenide (GaAs), the seed surface should havethree-fold rotational symmetry. Thus, GaN grows on the foreign substratewith the three-fold rotation symmetry, maintaining C-plane which hasalso the same symmetry. Thus, heteroepitaxy on a foreign substrate isrestricted to C-plane growth. There are two secondary important planesnext to C-plane.

[0041] One important plane is {1-100} planes which are vertical toC-plane. This is a cleavage plane. The {1-100} planes mean a set of sixindividual planes (1-100), (10-10), (01-10), (-1100), (-1010) and(0-110) which are all cleavage planes. The (1-100), (10-10), (01-10),(-1100), (-1010) and (0-110) planes are called “M-plane” for short. Thecleavage planes meet with each other at 60 degrees. Any two cleavageplanes are not vertical.

[0042] The other important plane is {11-20} planes which are vertical toC-plane. The {11-20} planes mean a set of six individual planes (11-20),(1-210), (-2110), (2-1-10), (-12-10) and (-1-120). The (11-20), (1-210),(-2110), (2-1-10), (-12-10) and (-1-120) planes are called “A-plane” forshort. A-planes are not cleavage planes. A-planes meet with each otherat 60 degrees.

[0043] C-plane {0001} is uniquely determined. But A-planes and M-planesare not uniquely determined, since A-planes and M-planes include threedifferent planes. Some of A-planes are vertical to some of M-planes.

[0044] All A-planes are vertical to C-plane. All M-planes are verticalto C-plane. Some of A-planes, some of M-planes and C-plane can build aset of orthogonal planes.

[0045] {circle over (6)} Japanese Patent Application No.10-147049(147049/'98) was filed by the same inventor as the presentinvention. {circle over (6)} proposed non-rectangular GaN devices whichhave sides of cleavage planes (M-planes). The GaN crystal has C-plane asa surface.

[0046] {circle over (7)} Japanese Patent Application No.11-273882(273882/'99) was filed by the same inventor as the presentinvention. Conventional growth means a growth by maintaining amirror-flat, even C-plane surface. {circle over (7)} proposedfacet-growth of GaN along c-axis which keeps various facets on C-plane.The facets on C-plane signify small other planes than C-plane. Facetsform hexagonal pits or hillocks and dodecagonal pits or hillocks onC-plane. Although GaN grows on an average along the c-axis, variousfacets cover a surface of growing GaN. The facets sweep dislocationsdown to the bottoms of the facet pits. Dislocations are effectivelyreduced by the facets.

[0047] {circle over (8)} Japanese Patent Application No.2000-207783(207783/'00) was filed by the same inventor as the presentinvention. {circle over (8)} discovered a fact that dislocations extendin parallel with the growing direction in a GaN crystal. C-plane growthprolongs dislocations in parallel with the c-axis. {circle over (8)}proposed a sophisticated method of growing a tall GaN crystal in thec-direction on a C-plane of a GaN seed, cutting the GaN crystal in Aplanes, obtaining an A-plane GaN seed crystal, growing the GaN crystalin an A-direction on the A-plane seed, cutting the A-grown GaN in Mplanes and obtaining M-plane GaN seeds with low dislocation density.Prior art of {circle over (1)} to {circle over (7)} grow C-plane GaNcrystals in the c-direction on a foreign material or a C-plane GaN seed.Only {circle over (8)} proposed non-C-plane growth of GaN on anon-C-plane GaN substrate.

[0048] All of the known attempts of on-sapphire GaN growth growC-surface GaN crystals having a C-surface as a top without exception.There is a reason of the absolute prevalence of C-plane GaN crystals.When a sapphire (α-Al₂O₃) single crystal substrate is used as asubstrate, a C(0001) surface sapphire is used to be chosen. Sapphirebelongs to a trigonal symmetry group which requires four indices forrepresenting orientations of planes and directions. GaN has hexagonalsymmetry. Lengths of c-axes are different between sapphire and galliumnitride. GaN has three typical, low index planes of A-plane, M-plane andC-plane, as explained before. A-plane or M-plane GaN cannot grow onA-plane or M-plane sapphire, because A-plane or M-plane are too complexto coincide with a similar plane of sapphire without misfit. OnlyC-plane of GaN can join on C-plane of sapphire. Thus, a smooth, flatC-plane GaN crystal can be easily grown on a C-plane sapphire substrate.All of the known InGaN-LEDs have a pile of C-plane GaN or InGaN layerson a C-plane sapphire substrate.

[0049] Similarly, in the case of GaAs substrates, a three-foldrotationally symmetric (111) GaAs substrate is selected as a substrate.GaAs belongs to a cubic symmetry group. But a (111) plane of GaAs hasthree-fold rotational symmetry. The (111) GaAs substrate allows onlyC-plane GaN having the corresponding rotation symmetry to grow on.Foreign materials as a substrate cannot grow non-C-plane GaN at all.

[0050] A thick bulk crystal requires far more amount of dopants than athin film crystal. The amount of dopants is in proportion to a thicknessor volume of grown crystals. Silane gas (SiH₄) is a dangerous gas whichsometimes induces a burst. An n-type GaN bulk crystal would require alarger amount of an n-dopant than an n-type thin film. The inventorsprefer oxygen to silicon as the n-dopant, because water (H₂O) as anoxygen compound is far safer than silane gas (SiH₄). The inventors triedto dope GaN bulk crystals with oxygen as an n-dopant. However, mirrorflat, C-plane GaN crystals cannot be doped enough with oxygen. Theinventors discovered a fact that oxygen does not go into GaN easily viaC-plane and C-plane repulses oxygen. The inventors found orientationdependence of oxygen doping for the first time. The inventors were awareof the fact that oxygen doping has orientation dependence. The inventorsnoticed that C-plane is the worst plane for oxygen doping.

[0051] The orientation dependence of oxygen doping is a novelphenomenon. It is not easy to understand the orientation dependence ofoxygen doping. Nobody found the phenomenon before the inventors of thepresent invention. The inventors analysed atomic components on a surfaceof C-plane grown GaN crystals by SIMS(Secondary Ion Mass Spectroscopy).The SIMS determines atomic ratios on a surface of an object sample byemitting first ions, accelerating the first ions, shooting the firstions at the sample for inducing secondary ions emitted out of thesample, analysing the mass of the secondary ions and counting thenumbers of the emitted secondary ions. The numbers of the emittedsecondary ions are proportional to products of emission efficiencies ofsecondary ions and atomic ratios on the object surface. Since theemission efficiencies are known parameters, the atomic ratios aredetermined.

[0052] At an early stage of the SIMS analysis, insufficient resolutionof the secondary ions and broadness of the first ion beam allowed a widesecondary beam to emanate from a wide area of the object. The broadsecondary ion beams obscured abnormality of oxygen doping. Then,C-planes of GaN seemed to emit oxygen secondary ions.

[0053] At a later stage, the inventors succeeded in converging the firstion beam and enhancing resolution of the SIMS. Narrow converged firstions and enhanced resolution revealed a surprising fact.

[0054] A C-plane surface of a C-grown GaN includes microscopic pits orhillocks. The microscopic hillocks and pits have many non-C-planes whichare called facets. A rough C-plane is an assembly of micro C-planes andmicro facets. Oxygen secondary ions were measured by discriminating themicro C-planes and micro non-C-plane facets. The inventors found thatthe oxygen secondary ions were emitted from the micro non-C-plane facetsand the micro C-planes do not emit the oxygen secondary ions. Namely,the micro C-planes included far smaller rate of oxygen than an averagerate. When oxygen concentration was 5×10¹⁸ cm⁻³ at non-C-plane facets,oxygen concentration was less than 1×10¹⁷ cm⁻³ at C-planes. The facetshave oxygen acceptance function which is more than 50 times as large asthat of C-planes. C-planes are the poorest in the function of acceptingoxygen. In the SIMS experiments, the secondary oxygen ions emanated notfrom C-planes but from the facets.

[0055] The inventors made a mirror-flat C-plane GaN crystal. The oxygenconcentration was less than 1×10¹⁷ cm⁻³ everywhere on a sample.

[0056] The SIMS analysis taught us that oxygen is hardly doped intoC-plane of GaN. The oxygen doping to a C-plane grown GaN crystal iscaused by the facets which have high acceptance function of oxygen.

[0057] When a GaN crystal grows in an average in the c-direction, oxygencan be doped into GaN via the facets. Facets enable C-plane GaN toaccept oxygen as an n-dopant. The oxygen accepting power is inproportion to the area of the facets. The wider the facets are, thestronger the oxygen acceptance power is. When C-plane is covered overallwith facets, the oxygen acceptance power attains to the maximum.

[0058] Otherwise, an A-plane GaN seed and an M-plane GaN seed are bestsubstrates for growing a non-C-plane GaN crystal and doping thenon-C-plane GaN crystal with oxygen.

[0059] In conclusion, Si which is a prevalent n-dopant for thin films ofGaN is not suitable for doping large GaN bulk crystals. Oxygen is asafer n-dopant. But conventional C-plane growth repulses oxygen. Oxygendoping is insufficient for the C-plane growth. An A-plane GaN seed, anM-plane seed and facet c-axis growth are effective to oxygen doping intoGaN crystals.

SUMMARY OF THE INVENTION

[0060] The inventors have investigated possibility of oxygen doping toGaN by growing various orientations of GaN crystals. The inventors havefound an orientation (plane) dependence of the oxygen doping to GaN. Anaccumulation of experiments taught the inventors that a flat C-plane isan unfavorable orientation but all the orientations of planes are notunfavorable to the oxygen doping. The inventors noticed an existence oforientations of GaN crystals except C-plane which are favorable toaccept oxygen as an n-dopant. There are two types (1) and (2) ofoxygen-acceptable planes.

[0061] (1) {kk-2kh} planes (k, h; integer)

[0062] {11-20} planes, in particular, accept oxygen with a high rate.{11-22} planes also show a high doping efficiency of oxygen. Oxygendoping efficiency is lower for higher numbers of indices of {kk-2kh}.

[0063] (2) {k-k0h} planes (k, h; integer)

[0064] {1-100} planes, in particular, are favorable planes for acceptingoxygen with a high rate. {1-101} planes also show a high dopingefficiency of oxygen. The higher the indices of {k-k0h} are, the lowerthe oxygen doping efficiency is.

[0065] Oxygen doping depends upon the orientation of planes. A {hkmn}plane has an inherent power for doping GaN with oxygen. The inherentpower of doping oxygen of the plane {hkmn} can be denoted by OD{fhkmn}.Detailed dependence OD{hkmn} is not clear yet. C-plane takes the minimumof OD. Thus, for any planes other than C-plane, OD{hkmn}>OD{0001}.

[0066] For special planes the oxygen doping power can be estimated bythe result of experiments.

[0067] A-planes {11-20} has more than 50 times as large power asC-plane; OD{11-20}>50OD{0001}.

[0068] M-planes {1-100} has more than 50 times as large power asC-plane; OD{1-100}>50OD{0001}.

[0069] Non-C-plane growth enables the growing GaN crystal to acceptoxygen effectively. Oxygen invades into the GaN crystal via non-C-planeson the surface. One favorable case is to grow GaN having overallnon-C-plane, for example, A-plane or M-plane. The other favorable caseis to grow faceted GaN in the c-axis with various facets of non-C-planeson the surface. Oxygen is absorbed via non-C-planes on the surface ofgrowing GaN more effectively than C-plane. The orientation dependence ofoxygen doping has been recently discovered by the inventors. The detailsare not clear for the inventors yet. Coupling bonds appearing out of acrystal are changed by orientation of the surface. Elements to becoupled to the bonds are varied by the surface orientation. Thus,impurity doping may have dependence upon the orientation of the surface.

[0070] A (0001) Ga surface of GaN has high resistance against invasionof oxygen to nitrogen sites as an n-impurity. The inventors confirmedthat the orientation dependence appears for all GaN growth upon sapphiresubstrates, silicon carbide substrates, gallium nitride substrates andso on.

BRIEF DESCRIPTION OF THE DRAWINGS

[0071]FIG. 1 is a sectional view of an HVPE (hydride vapor phaseepitaxy) apparatus for growing a gallium nitride crystal.

[0072]FIG. 2 is sectional views of GaN crystals of steps of Embodiment 1for making an oxygen doped GaN single crystal in vapor phase on anM(1-100)-plane GaN single crystal seed. FIG. 2(a) is a section of theM(1-100)-plane GaN seed. FIG. 2(b) is a section of the M-plane GaN seedand a (1-100) GaN crystal homoepitaxially grown on the M(1-100)-planeGaN seed. FIG. 2(c) is a section of the grown (1-100) GaN crystal fromwhich the seed crystal has been removed by etching. FIG. 2(d) is asection of a polished grown (1-100) GaN crystal with an M-plane surface.

[0073]FIG. 3 is sectional views of GaN crystals of steps of ComparisonExample 1 for making a GaN single crystal in vapor phase on aC(0001)-plane GaN single crystal seed. FIG. 3(a) is a section of theC(0001)-plane GaN seed. FIG. 3(b) is a section of the C-plane GaN seedand a (0001) GaN crystal homoepitaxially grown on the C(0001)-plane GaNseed. FIG. 3(c) is a section of the grown (0001) GaN crystal from whichthe seed crystal has been removed by etching. FIG. 3(d) is a section ofa polished grown (0001) GaN crystal with a C-plane surface.

[0074]FIG. 4 is sectional views of GaN crystals of steps of Embodiment 2for making an oxygen doped n-type GaN single crystal in vapor phase bymaintaining various non-C-plane facets on a C(0001)-plane GaN singlecrystal seed. FIG. 4(a) is a section of the C(0001)-plane GaN seed. FIG.4(b) is a section of the C-plane GaN seed and a facet-growing (0001) GaNcrystal grown on the C(0001)-plane GaN seed homoepitaxially in thec-axis <0001>. FIG. 4(c) is a section of the facet-growing (0001) GaNcrystal from which the seed crystal has been removed by etching. FIG.4(d) is a section of a polished grown (0001) GaN crystal with a C-planesurface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0075] The most effective method for doping gallium nitride crystal withoxygen is that water is added into material gases. In the case of anHVPE method, water is added into ammonia (NH₃) or hydrochloric (HCl)gases. When NH₃ and HCl inherently include water as an impurity, waterneeds not be added into the material gases. In this case, the GaNcrystal is consequently doped with oxygen of water existing inherentlyin the gases. But, in order to dope with oxygen stably, it is desirablethat a fixed bit amount of water is added into the gases.

[0076] There are two alternative methods of doping a gallium nitridecrystal substrate with oxygen effectively in accordance with theteaching of the present invention. One (A) is a non-C-plane method whichgrows gallium nitride upon a seed crystal having a non-C-plane surface.The other (B) is a C-plane facet-growing method which grows galliumnitride upon a seed crystal having a C-plane surface by maintaining manyvariously-oriented facets on the c-axis grown crystal upon the seedcrystal. The two are operative applications of the present invention.

[0077] (A) Non-C-plane Method

[0078] Method (A) employs a gallium nitride seed crystal having a {hkmn}surface except C plane {0001}, grows a {hkmn} faced gallium nitridecrystal on the non-C-plane {hkmn} surface of the seed crystal andobtains a single crystal ingot extending in the same direction <hkmn> asthe {hkmn} surface seed crystal.

[0079] Method (A) starts from a non-C-plane {hkmn} oriented GaN singlecrystal seed, wherein {hkmn}≠{0001}. Method (A) maintains thenon-C-plane {hkmn} on the surface of the growing crystal for injectingoxygen via the non-C-plane to the growing non-C-plane crystal. Oxygen iseffectively supplied overall into the growing non-C-plane galliumnitride crystal.

[0080] For example, a suitable candidate seed is a {1-100} plane(M-plane) gallium nitride crystal. In general, {k-k0h}-plane galliumnitride crystals are candidates for the seeds for applying Method (A).

[0081] For example, another preferable candidate seed is a {11-20} plane(A-plane) gallium nitride crystal. In general, {kk-2kh}-plane galliumnitride crystals are candidates for the seeds for applying Method (A).

[0082] In Method (A), the oxygen doping efficiency OD can be expressedas a function of the orientation indices simply by

OD=OD{hkmn}.

[0083] Method (A) is based upon a simple principle of selective oxygendoping on the non-C-planes. Method (A), however, has still a problem forcarrying out. There is no natural gallium nitride single crystal havinga non-C-plane surface. Method (A) requires synthesis of a non-C-planegallium nitride single crystal as a seed by some means. No vapor phaseheteroepitaxial growth upon a foreign material substrate can produce anon-C-plane gallium nitride crystal. At present, all of the GaN or GaInNfilms which are heteroepitaxially produced upon sapphire substrates of aC-plane surface with three-fold rotational symmetry for making blueLEDs, are C-plane GaN or InGaN films. No non-C-plane GaN crystal can beheteroepitaxially made upon a sapphire substrate in any cases. Whenheteroepitaxy makes a GaN crystal upon a sapphire substrate, it isimpossible to obtain a freestanding isolated GaN crystal due to thedifficulty of eliminating the sapphire substrate from the GaN/sapphirecrystal. Sapphire is not suitable for a seed crystal of making afreestanding gallium nitride crystal having a non-C-plane surface.

[0084] Instead of sapphire, a gallium arsenide (111) crystal is apromising candidate for producing a gallium nitride crystal as a seed. A(111) gallium arsenide (GaAs) substrate enables vapor phase lateralovergrowth to make a gallium nitride (GaN) crystal growing along ac-axis. Unlike sturdy sapphire, the GaAs substrate can be eliminatedfrom the GaN crystal by aqua regia. Thus, a freestanding GaN singlecrystal can be obtained by heteroepitaxy upon a GaAs substrate. However,the freestanding GaN crystal produced upon the (111) GaAs substrate isalso a C-plane GaN single crystal having a C-plane surface. Thus,A-plane {11-20} surface GaN single crystals as seeds are obtained bygrowing another tall GaN single crystal in the c-directionhomoepitaxially on the C-surface GaN single crystal and slicing thenewly-grown GaN single crystal in {11-20} planes which are vertical toC-plane. The {11-20} surface crystals can be employed as a seed forgrowing a n-type GaN crystal doped with oxygen. In general, {kk-2kh}plane crystals can be made by heteroepitaxially growing a C-plane GaNsingle crystal on a (111) GaAs substrate, eliminating the GaAssubstrate, homoepitaxially growing a thick (tall) C(0001)-plane GaNsingle crystal on the C-plane GaN substrate and cutting the thickC-plane GaN in the {kk-2kh} plane. Thus, Method (A) requires doublesteps of growing GaN single crystals for preparing a seed.

[0085] (B) C-plane Facet-Growing Method

[0086] Method (B) employs a C-plane gallium nitride seed crystal or aforeign material crystal having three-rotational symmetry, grows theC-plane {0001} gallium nitride crystal having many facets ofnon-C-planes {hkmn} in vapor phase by supplying material gases includingoxygen components, maintains the facets during the growth, obtains aC-plane faceted single crystal ingot extending in the c-axis direction<0001>, polishes the surface of the facetted C-plane crystal, eliminatesthe superficial facets from the surface of the GaN single crystal, andobtains an oxygen doped n-type C-plane GaN single crystal. The averageplane of the growing crystal is the C-plane. But, the surface is coveredwith many facets having various planes {k-k0h}, {kk-2kh} and so on. Theratio occupied by C-planes is small on the growing surface. AlthoughC-planes reject oxygen, facets of various orientations effectivelyabsorb oxygen.

[0087] In practice, favorable facet planes for allowing oxygen dopingare {k-k0h} (k and h are integers) planes, in particular, {1-101}planes. These facets are obtained by inclining M {1-100} planes. Since M{1-100} planes are vertical to C {0001} plane, M-planes cannot be facetson C {0001} plane.

[0088] Other favorable facet planes for inducing oxygen doping are{kk-2kh} (k and h are integers) planes, in particular, {11-22} planes.These facets are obtained by inclining A {11-20} planes. Since A {11-20}planes are vertical to C {0001} plane, A-planes cannot be facets on C{0001} plane. These are the case of including only a single kind offacets.

[0089] A GaN single crystal has six-fold rotational symmetry. A singlekind of facets is a set of six different individual planes. For example,a facet representation {kk-2kh} contains (kk-2kh), (kk-2k-h), (-2kkkh),(-2kkk-h), (k-2kkh) and (k-2kk-h). A single kind of facets can makehexagonal conical hills or hexagonal conical pits on a growing C-plane.Sometimes some of the six facets disappear. In the case, irregular pitsor hills of trigonal, rhombic, or pentagonal cones appear on the growingC-plane. The facets absorb oxygen.

[0090] Sometimes two kinds of facets coexist on the growing C-plane.Coexistence of various kinds of facets is more favorable for absorbingoxygen from material gases. In the present invention, oxygen doping isaccelerated by maintaining two kinds of facets {kk-2kh} and {k-k0h} onthe growing C-plane. For example, six facets of {1-101} and six facetsof {11-22} can build regular dodecagonal conical pits or hillocks whichare effective to absorb oxygen. An assembly of more than two kinds offacets makes complex pits or hillocks which raise the efficiency ofoxygen doping. There is little fraction of C-planes on thefacet-covered, rugged GaN surface growing in the c-direction.

[0091] When two kinds of facets {kk-2kh} and {k-k0h} accompany thec-direction GaN growth, hexagonal cone pits and dodecagonal cone pitsenhance the probability of absorbing oxygen. The growing GaN isconverted into an n type semiconductor by being doped with oxygen. Thetwo kind facet method has a complex feature. The total efficiency OD ofoxygen doping is a sum of the contributions from the different facets{hkmn}.

OD=Σρ{hkmn}OD{hkmn}.

[0092] Here, ρ{hkmn} signifies the existence probability of the facet{hkmn} on the faceted C-plane.

[0093] Oxygen doped facetted gallium nitride (GaN) single crystals aremade upon gallium arsenide (GaAs) substrates by all the methods of theHVPE method, the MOC method, the MOCVD method and the sublimation methodwhich have been employed for growing thin GaN films upon sapphire(Al₂O₃) substrates.

[0094] [EMBODIMENT 1 (M (1-100) surfaced GaN crystal seed; FIG. 2)]

[0095] An M (1-100) plane GaN single crystal is prepared as a seed forputting Method (A) into practice by slicing a bulk GaN single crystal inthe planes parallel with an M (1-100) plane (FIG. 2(a)). The M-planeseed crystal has a (1-100) top surface and a (-1100) bottom surface. Thebulk GaN single crystal was obtained by growing a thick GaN singlecrystal upon a (111) GaAs substrate by a lateral overgrowth method andremoving the GaAs substrate by solving with aqua regia. The slicingdirection M (1-100) is one of planes parallel with a growth direction<0001>. The sliced M-surface crystal is polished. The prepared M-planeseed crystal has a flat smooth surface which is immune from degradedsuperficial layers.

[0096] An HVPE (hydride vapor phase epitaxy) method grows a galliumnitride crystal on the M-plane GaN seed crystal under the followingcondition. growth temperature 1020° C. NH₃ partial pressure 0.2 atm (2 ×10⁴ Pa) HCl partial pressure 1 × 10⁻² atm (10³ Pa) growth time 6 hoursGaN layer thickness about 500 μm

[0097] The NH₃ material gas includes 2 ppm of water (H₂O). Water isadded to the NH₃ gas intentionally as an oxygen source. A (1-100) planegallium nitride crystal of 500 μm in thickness is obtained, as shown inFIG. 2(b). The bottom seed GaN is removed away by polishing, as shown inFIG. 2(c). The M-plane grown crystal is evenly polished. The polishedGaN crystal as shown in FIG. 2(d) has a thickness of about 400 μm.

[0098] Electric properties of the grown M-plane-GaN crystal areinvestigated by Hall's measurement. The Hall measurement confirms thatcarriers are electrons. Average values of the Hall measurement at fourpoints on the GaN crystal are,

[0099] carrier density=6×10¹⁸ cm⁻³

[0100] carrier mobility=160 Vs/cm².

[0101] The carrier density and mobility are uniform overall on thesurface of the GaN crystal.

[0102] Element ratios existing at the surface of the grown GaN sampleare analysed by the SIMS (secondary ion mass spectroscopy). Theconcentrations of elements are, hydrogen (H) 2 × 10¹⁷ cm⁻³ carbon (C) 3× 10¹⁶ cm⁻³ oxygen (O) 8 × 10¹⁸ cm⁻³ silicon (Si) 3 × 10¹⁷ cm⁻³.

[0103] Since the carriers are electrons and the carrier density (freeelectron density) is 6×10¹⁸ cm⁻³, the grown GaN is an n-typesemiconductor. N-type impurities which have a probability of giving freeelectrons to the GaN crystal are oxygen (O), carbon (C) and silicon(Si). Since an n-type dopant gives one electron to the matrix crystal,the concentration of the n-type dopant should be higher than the carrierdensity. The carbon concentration (3×10¹⁶ cm⁻³) is lower than thecarrier density (6×10¹⁸ cm⁻³). The silicon concentration (3×10¹⁷ cm⁻³)is lower than the carrier density (6×10¹⁸ cm⁻³).

[0104] Only the oxygen concentration (8×10¹⁸ cm⁻³) is higher than thecarrier density (6×10¹⁸ cm⁻³). This fact means that the free carriers(electrons) derive from oxygen atoms, oxygen acts as an n-type dopantand oxygen has a high activation rate (about 0.8) in the GaN crystal.

[0105] Electrical conductivity is measured. The resistivity which is aninverse of conductivity is 7×10⁻³ Ωcm. The grown GaN crystal has a highconductivity of 140/Ωcm. The high conductivity enables the GaN singlecrystal to act as an n-type GaN substrate on which LDs or LEDs arefabricated. The known insulating sapphire substrates are suffering fromthe difficulty of non-cleavage and the difficulty of an n-electrode. Thegrown GaN substrate has good cleavage and good conduction which allows abottom n-electrode.

[0106] The grown GaN substrate of this embodiment is a freestanding GaNsubstrate having a flat, smooth surface and a thickness of 400 μm. Thisn-type GaN substrate can be used as a substrate of devices byepitaxially piling layers thereon.

[0107] [COMPARISON EXAMPLE 1 (Flat C-plane (0001) GaN Growth; FIG. 3)]

[0108] A C (0001) plane GaN single crystal is prepared as a seed byslicing a bulk GaN single crystal in the planes parallel with a C (0001)plane (FIG. 3(a)) for making a comparison between C-plane growth andM-plane growth. The C-plane seed crystal has a (0001) top surface and a(000-1) bottom surface. The top layer of the C-plane is a Ga layer,which is sometimes represented by (0001)Ga. The bottom layer of theC-plane is an N layer, which is sometimes represented by (0001)N. Thesliced C-surface crystal is polished. The prepared C-plane seed crystalhas a flat smooth surface which is immune from degraded superficiallayers.

[0109] A flat, even, facetless C(0001)-plane GaN crystal is grown on theC-plane GaN seed crystal by an HVPE (hydride vapor phase epitaxy) methodunder the following condition. growth temperature 1050° C. NH₃ partialpressure 0.15 atm (1.5 × 10⁴ Pa) HCl partial pressure 5 × 10⁻³ atm (5 ×10² Pa) growth time 10 hours GaN layer thickness about 500 μm

[0110] The NH₃ material gas includes 2 ppm of water (H₂O). Water isadded to the NH₃ gas intentionally as an oxygen source. The HVPE methodmaintains a flat, even surface of the growing C-plane GaN crystal. Aflat C(0001) plane gallium nitride crystal of 500 μm in thickness isobtained, as shown in FIG. 3(b). The top surface is a smooth, mirror(0001) plane. The bottom seed GaN is removed away by polishing, as shownin FIG. 3(c). The C-plane grown crystal is evenly polished. The polishedGaN crystal as shown in FIG. 3(d) has a thickness of about 400 μm.

[0111] The Hall measurement for determining carrier density failed inthe C-GaN Comparison Example. The C-plane GaN specimen has too highresistivity and too low conductivity. The conductivity cannot bemeasured at any spots upon the C-GaN specimen by the measurement toolsavailable for the inventors. The C-plane GaN is an insulator which ispoor in free carriers. The C-GaN is neither n-type nor p-type but anintrinsic-type semiconductor with high resistance. The SIMS (SecondaryIon Mass Spectroscopy) measurement shows the ratios of elements on thetop surface of the C-GaN, hydrogen (H)   1 × 10¹⁸ cm⁻³ carbon (C)   7 ×10¹⁶ cm⁻³ oxygen (O)   1 × 10¹⁷ cm⁻³ silicon (Si) <2 × 10¹⁶ cm⁻³.

[0112] Comparison Example 1 has far smaller oxygen concentration thanEmbodiment 1, although NH₃ gas includes the same rate (2 ppm) of water.The oxygen concentration (1×10¹⁷ cm⁻³) in Comparison Example 1 is aboutone hundredth ({fraction (1/100)}) of Embodiment 1 (8×10¹⁸ cm⁻³). Thecarbon concentration (7×10¹⁶ cm⁻³) is about twice as high as Embodiment1 (3×10¹⁶ cm⁻³). The silicon concentration (<2×10¹⁶ cm⁻³) is reduceddown to one tenth of Embodiment 1 (3×10¹⁷ cm⁻³). The difference resultsfrom the difference of the growing surfaces (the M plane for Embodiment1 and the C plane for Comparison Example 1). The C-plane growth seems toenhance the absorption of carbon (C) and hydrogen (H). The C-planegrowth seems to suppress the doping of silicon (Si) and oxygen (O).Orientation dependence of silicon is smaller than that of oxygen. Oxygenexhibits the most conspicuous orientation dependence of doping.

[0113] Comparison Example 1 which is made by the flat C-plane growthcannot absorb oxygen effectively. Poor oxygen intake incurs poor freeelectrons. Thus, the even C-plane growth GaN crystal becomes aninsulator. The insulating GaN crystal is not suitable for a substratefor making GaN devices, because an n-electrode cannot form on the bottomof the substrate.

[0114] [Embodiment 2 (Faceted C-plane (0001) GaN Growth; FIG. 4)]

[0115] A C (0001) plane GaN single crystal is prepared as a seed byslicing a bulk GaN single crystal in the planes parallel with a C (0001)plane (FIG. 4(a)). The C-plane seed crystal has a (0001) top surface anda (000-1) bottom surface. The top surface of the GaN seed is a (0001)Gaplane like Comparison Example 1. The sliced C-surface crystal ispolished. The prepared C-plane seed crystal has a flat smooth surfacewhich is immune from degraded superficial layers.

[0116] A faceted GaN crystal is grown in the c-direction on the C-planeGaN seed by an HVPE (hydride vapor phase epitaxy) method withouteliminating facets under the following condition. growth temperature1030° C. NH₃ partial pressure 0.2 atm (2 × 10⁴ Pa) HCl partial pressure1 × 10⁻² atm (10³ Pa) growth time 5 hours GaN layer thickness about 500μm

[0117] The NH₃ material gas includes 2 ppm of water (H₂O). Water isadded to the NH₃ gas intentionally as an oxygen source. The HVPE methodmaintains faceted surface containing various pits or hillocks on thegrowing C-plane GaN crystal. A lower temperature of 1030° C., a higherNH₃ partial pressure of 0.2 atm and a higher HCl partial pressure of1×10⁻² atm enable a C-plane GaN crystal to maintain facet-growth. Arugged C(0001) plane gallium nitride crystal of 500 μm in thickness isobtained, as shown in FIG. 4(b). The top surface is a rugged (0001)plane covered with many facets of various orientations except a (0001)plane. Almost all of the top is occupied by the facets. The grown GaN isglittering by the facets. Plenty of hexagonal conical pits anddodecagonal conical pits are found on the GaN crystal. The ratio ofC-planes is very small on the top surface.

[0118] The facets on the top include various orientations. Prevalentfacets are {1-101} planes, {11-22} planes, {1-102} planes, {11-24}planes and so on. These facets can be represented collectively by{k-k0h} planes (k, h: integer) or {kk-2kh} planes (k, h: integer).

[0119] The bottom (0001) seed GaN is removed away by polishing, as shownin FIG. 4(c). The average thickness is about 400 μm. The top of thefacet-grown crystal is evenly polished for removing the facets. Thepolished GaN (0001) crystal as shown in FIG. 4(d) has a thickness ofabout 350 μm.

[0120] Electrical properties are investigated by Hall measurement atfour spots of the GaN crystal of Embodiment 2. Averages at the fourpoints are,

[0121] carrier density=5×10¹⁸ cm⁻³

[0122] carrier mobility=170 Vs/cm².

[0123] The SIMS (Secondary Ion Mass Spectroscopy) measurement shows theratios of elements on the top surface of the C-GaN, hydrogen (H)   2 ×10¹⁷ cm⁻³ carbon (C)   3 × 10¹⁶ cm⁻³ oxygen (O)   5 × 10¹⁸ cm⁻³ silicon(Si) <4 × 10¹⁶ cm⁻³.

[0124] Embodiment 2 has 5×10¹⁸ cm⁻³ oxygen concentration and 5×10¹⁸ cm⁻³carrier concentration. The oxygen concentration is equal to the carrierconcentration. The oxygen concentration (5×10¹⁸ cm⁻³) of Embodiment 2 is50 times as much as Comparison Example 1 (1×10¹⁷ cm⁻³). Si, C and O havea probability of acting an n-dopant in GaN. But the siliconconcentration (<4×10¹⁶ cm⁻³) and the carbon concentration (3×10¹⁶ cm⁻³)are far smaller than the carrier concentration (5×10¹⁸ cm⁻³). The factconfirms that free electrons of 5×10¹⁸ cm⁻³ are generated by oxygenatoms as an n-dopant with a high activation rate nearly equal to 100%.

[0125] The difference of oxygen concentration between Comparison Example1 and Embodiment 2 results from the difference of the facet-growth orthe non-facet, mirror flat growth in the c-direction. The flat C-planegrowth seems to suppress the doping of oxygen. The faceted C-planegrowth allows microscopic facets to absorb oxygen atoms effectively. Themany microscopic {kk-2kh} facets or {k-k0h} facets can absorb oxygenatoms like the allover M-plane of Embodiment 1. Orientation dependenceof oxygen-doping is strong. C-planes are inherently the poorest planefor doping with oxygen. But, the faceted C-plane growth can dope a GaNcrystal with oxygen at a high rate, because rugged C-plane includes manyfacets which accelerate oxygen doping.

[0126] Electrical conductivity is measured. The resistivity which is aninverse of conductivity is 6×10⁻³ Ωcm. The faceted C-grown GaN crystalhas a high conductivity of about 170/Ωcm. The high conductivity enablesthe GaN single crystal to act as an n-type GaN substrate on which LDs orLEDs are fabricated. The known insulating sapphire substrates aresuffering from the difficulty of non-cleavage and the difficulty of ann-electrode. The present invention gives a Si-undoped, n-type GaN bulksingle crystal substrate which is doped with oxygen and has goodcleavage and good conduction which allows a bottom n-electrode.

[0127] The faceted C-grown GaN substrate of Embodiment 2 is afreestanding GaN substrate having a flat, smooth surface and a thicknessof 350 μm after the facets have been removed by polishing. The obtainedn-type GaN substrate is available for a substrate of devices byepitaxially piling layers thereon.

What we claim is,
 1. An oxygen doping method to a gallium nitride singlecrystal substrate, comprising the steps of: preparing a non-C-planegallium nitride single crystal seed having a surface except C-plane,supplying the non-C-plane gallium nitride seed with material gasesincluding a gallium material, a nitrogen material and an oxygen materialwithout silicon, growing a gallium nitride bulk crystal upon thenon-C-plane gallium nitride seed in vapor phase, maintaining thenon-C-plane surface on the growing gallium nitride bulk crystal, anddoping the growing gallium nitride bulk crystal with oxygen via thenon-C-plane surface.
 2. The method according to claim 1, wherein thenon-C-plane surface is {kk-2kh} planes (k,h; integer) and the {kk-2kh}planes are maintained during the growth as the surface of the growinggallium nitride bulk crystal for doping the crystal with oxygen via the{kk-2kh} planes.
 3. The method according to claim 1, wherein thenon-C-plane surface is {k-k0h} planes (k,h; integer) and the {k-k0h}planes are maintained during the growth as the surface of the growinggallium nitride bulk crystal for doping the crystal with oxygen via the{k-k0h} planes.
 4. The method according to claim 2, wherein thenon-C-plane surface is {11-20} planes (A-planes) and the {11-20} planesare maintained during the growth as the surface of the growing galliumnitride bulk crystal for doping the crystal with oxygen via the {11-20}planes.
 5. The method according to claim 3, wherein the non-C-planesurface is {1-100} planes (M-planes) and the {1-100} planes aremaintained during the growth as the surface of the growing galliumnitride bulk crystal for doping the crystal with oxygen via the {1-100}planes.
 6. An oxygen doping method to a gallium nitride single crystalsubstrate, comprising the steps of: preparing a foreign material singlecrystal seed having three-fold rotational symmetry or a C-plane galliumnitride single crystal seed having a C-plane surface, supplying theforeign material seed or the C-plane gallium nitride seed with materialgases including a gallium material, a nitrogen material and an oxygenmaterial without silicon, growing a gallium nitride bulk crystal withfacets having non-C-planes upon the foreign material seed or the C-planegallium nitride seed in vapor phase, maintaining the facets having thenon-C-planes on the growing gallium nitride bulk crystal, and doping thegrowing gallium nitride bulk crystal with oxygen via the non-C-planes ofthe facets.
 7. The method according to claim 6, wherein the non-C-planesof the facets are {kk-2kh} planes (k,h; integer) and the {kk-2kh} planesare maintained during the growth as the facets on the growing galliumnitride bulk crystal for doping the crystal with oxygen via the {kk-2kh}planes.
 8. The method according to claim 6, wherein the non-C-planes ofthe facets are {k-k0h} planes (k,h; integer) and the {k-k0h} planes aremaintained during the growth as the facets on the growing galliumnitride bulk crystal for doping the crystal with oxygen via the {k-k0h}planes.
 9. The method according to claim 7, wherein the non-C-planes ofthe facets are {11-21} and {11-22} planes and the {11-21} and {11-22}planes are maintained during the growth as the facets on the growinggallium nitride bulk crystal for doping the crystal with oxygen via the{11-21} and {11-22} planes.
 10. The method according to claim 8, whereinthe non-C-planes of the facets are {1-101} planes and the {1-101} planesare maintained during the growth as the facets on the growing galliumnitride bulk crystal for doping the crystal with oxygen via the {1-101}planes.
 11. An oxygen doped n-type gallium nitride freestanding singlecrystal substrate having oxygen atoms as an n-dopant and a non-C-planesurface, produced by the steps of: preparing a non-C-plane galliumnitride single crystal seed having a surface except C-plane, supplyingthe non-C-plane gallium nitride seed with material gases including agallium material, a nitrogen material and an oxygen material withoutsilicon, growing a gallium nitride bulk crystal upon the non-C-planegallium nitride seed in vapor phase, maintaining the non-C-plane surfaceon the growing gallium nitride bulk crystal, doping the growing galliumnitride bulk crystal with oxygen via the non-C-plane surface, andeliminating the non-C-plane gallium nitride single crystal seed from thegrown gallium nitride bulk crystal by etching or polishing.
 12. Thegallium nitride single crystal substrate according to claim 11, whereinthe non-C-plane surface is {kk-2kh} planes (k,h; integer).
 13. Thegallium nitride single crystal substrate according to claim 11, whereinthe non-C-plane surface is {k-k0h} planes (k,h; integer).
 14. Thegallium nitride single crystal substrate according to claim 12, whereinthe non-C-plane surface is {11-20} planes.
 15. The gallium nitridesingle crystal substrate according to claim 13, wherein the non-C-planesurface is {1-100} planes.
 16. An oxygen doped n-type gallium nitridefreestanding single crystal substrate having oxygen atoms as an n-dopantand a C-plane surface, produced by the steps of: preparing a foreignmaterial single crystal seed having a three-fold rotational symmetry ora C-plane gallium nitride single crystal seed having a C-plane surface,supplying the foreign material seed or the C-plane gallium nitride seedwith material gases including a gallium material, a nitrogen materialand an oxygen material without silicon, growing a gallium nitride bulkcrystal with facets having non-C-planes upon the foreign material seedor the C-plane gallium nitride seed in vapor phase, maintaining thefacets having the non-C-planes on the growing gallium nitride bulkcrystal, doping the growing gallium nitride bulk crystal with oxygen viathe non-C-planes of the facets, eliminating the foreign material seed orthe C-plane gallium nitride single crystal seed from the grown galliumnitride bulk crystal by etching or polishing, and polishing the facetedsurface of the grown gallium nitride bulk crystal for eliminating thefacets.
 17. The gallium nitride single crystal substrate according toclaim 16, wherein the non-C-planes of the facets are {kk-2kh} planes(k,h; integer) and the {kk-2kh} planes are maintained during the growthas the facets on the growing gallium nitride bulk crystal for doping thecrystal with oxygen via the {kk-2kh} planes.
 18. The gallium nitridesingle crystal substrate according to claim 16, wherein the non-C-planesof the facets are {k-k0h} planes (k,h; integer) and the {k-k0h} planesare maintained during the growth as the facets on the growing galliumnitride bulk crystal for doping the crystal with oxygen via the {k-k0h}planes.
 19. The gallium nitride single crystal substrate according toclaim 17, wherein the non-C-planes of the facets are {11-21} and {11-22} planes and the {11-21} and {11-22} planes are maintained duringthe growth as the facets on the growing gallium nitride bulk crystal fordoping the crystal with oxygen via the {11-21} and {11-22} planes. 20.The gallium nitride single crystal substrate according to claim 18,wherein the non-C-planes of the facets are {1-101} planes and the{1-101} planes are maintained during the growth as the facets on thegrowing gallium nitride bulk crystal for doping the crystal with oxygenvia the {1-101} planes.